1. Field of the Invention
The present invention relates to a fuel cell system built as a system in which a fuel cell and an electric storage device are provided in parallel.
2. Description of Related Art
In recent years, there have been developed various types of fuel cell systems built as a system in which a fuel cell and an electric storage device are provided in parallel (for example, see JP-A-2004-71260). An example of the structure of a conventional fuel cell system is shown in FIG. 6.
The conventional fuel cell system shown in FIG. 6 is a system in which a fuel cell and an electric storage device are provided in parallel, and is provided with a fuel cell stack 1, a fuel feeder 2, a rechargeable battery 3, which is an electric storage device, a blocking diode D1, a DC/DC converter 5′ for rechargeable battery (hereinafter referred to as a rechargeable battery DC/DC converter 5′), and a system output terminal 7. The system output terminal 7 is a direct current output terminal composed of a positive electrode terminal and a negative electrode terminal.
The fuel feeder 2 supplies the fuel cell stack 1 with a predetermined amount of fuel at regular intervals, and recovers the fuel that has not been consumed by the fuel cell stack 1. The fuel cell stack 1 has a positive electrode output end connected to a positive electrode terminal of the system output terminal 7 through the blocking diode D1. The rechargeable battery 3 is connected to an input end of the rechargeable battery DC/DC converter 5′, and a positive electrode output end of the rechargeable battery DC/DC converter 5′ is connected to the positive electrode terminal of the system output terminal 7. Though not shown in the drawing, negative electrode output ends of the fuel cell stack 1 and the rechargeable battery DC/DC converter 5′ are connected to a negative electrode terminal of the system output terminal 7. The fuel cell system shown in FIG. 6 operates with electric power derived from the output of the fuel cell system, and, during system start-up, makes the fuel feeder 2 operate with the output of the rechargeable battery 3.
When the system output terminal 7 is connected to a direct current input terminal of an electric appliance (a load), electric power is supplied from the conventional fuel cell system shown in FIG. 6 to the electric appliance. In general, specifications of the fuel cell stack 1, the fuel feeder 2, the rechargeable battery 3, and the rechargeable battery DC/DC converter 5′ are individually determined according to the electric appliance, and a voltage suitable for the electric appliance is outputted from the system output terminal 7. For example, when a direct current input terminal of the electric appliance is a 16V-direct current input terminal, the specifications of the fuel cell stack 1, the fuel feeder 2, the rechargeable battery 3, and the rechargeable battery DC/DC converter 5′ are individually determined so that a voltage outputted from the system output terminal 7 becomes approximately 16 V.
As described above, the conventional fuel cell system shown in FIG. 6 has a specification tailored to a particular electric appliance (for example, an electric appliance having a 16 V-direct current input terminal), and thus cannot be offered as a versatile fuel cell system.
Here, a specification of the rechargeable battery 3 may be modified by using a rechargeable battery built in an electric appliance (a load) to be connected to the system output terminal 7 as the rechargeable battery 3 of the conventional fuel cell system shown in FIG. 6. In this case, however, it is necessary to provide the electric appliance with an extra direct current output terminal for outputting an output voltage of the rechargeable battery thereof, and thus inconveniently leads to problems such as increased cost of the electric appliance or inapplicability to existing electric appliances.